American Scientist

As planets are being discovered around other stars by the thousands, several scientific disciplines, including astronomy, planetary science, and biochemistry, are converging, with the goal of locating and identifying life elsewhere in the Universe. We are engaged in a search for habitability—conditions suitable for life—even though we lack a clear definition of what life is. We are hunting for something we cannot yet sharply define. Nevertheless, we can make informed inferences about what life requires. From what we know about life on Earth, liquid water appears to be an essential ingredient. If an exoplanet orbits at the appropriate range of distances from its star to allow liquid water to exist on its surface, then it is said to be in the habitable zone— it is not too hot, not too cold, purportedly just right for living things.

The notion of the habitable zone has its origins in what we know about the planetary trio of Venus, Earth, and Mars. With surface temperatures exceeding 400 degrees Celsius, present-day Venus is a scorching inferno largely devoid of water. Its hostile temperatures are a direct consequence of its thick atmosphere being dominated by carbon dioxide—a powerful greenhouse gas that makes up less than one part in a thousand of the atmosphere of Earth. Mars is a study in contrasts, having a thin atmosphere, large temperature swings, and an average surface temperature that is well below the freezing point of water. Earth sits between these two extremes, so it is convenient to visualize Earth as residing within a zone of habitability, flanked by Venus and Mars.

With the flurry of recent discoveries made with the Kepler Space Telescope, it is routine to encounter media reports of “habitable-zone exoplanets”—sometimes accompanied by speculation on what types of life forms may exist on them—using a conception of the habitable zone that extrapolates directly from what we know about our own Solar System. The habitable zone is a star-specific concept, however. Stars exist in a variety of sizes and masses. More massive stars tend to burn more brightly but have shorter lifetimes. The most common types of stars in the Universe are not like our Sun, but instead have masses between 10 and 50 percent of it. These red dwarfs have cooler temperatures than our Sun and radiate far less energy, which means that if a planet of Earth’s size were to maintain the same range of atmospheric temperatures it would have to orbit such stars more closely.

A Problem of the Atmosphere

The habitable zone is also an atmosphere-specific concept. Three types of atmospheric gases strongly influence a body’s surface temperature. First, we need an incondensible greenhouse gas —one that stays in its gaseous form over the range of temperatures found in the atmosphere. On Earth, this role is played by carbon dioxide. Second, we need a condensible greenhouse gas, which exists in both gaseous and liquid forms. Water is the condensible greenhouse gas of our atmosphere and is the lynchpin of the hydrological cycle.

The boundaries of the habitable zone are determined by what happens to the condensible and incondensible greenhouse gases at different distances from the parent star. The inner boundary of the habitable zone is the distance at which the condensible greenhouse gas cannot condense, and the outer boundary of the habitable zone is the distance at which the incondensible greenhouse gas can condense. If the Earth were located too close to the Sun, then higher temperatures would result in more water existing as vapor, which in turn would lead to further warming. The planet would compensate for this greenhouse warming by emitting more infrared radiation and by shedding heat, but at some point there would be so much water vapor in the atmosphere that it would become opaque to infrared radiation. At that point, the cooling of the atmosphere would be overwhelmed by heating, leading to a runaway greenhouse effect. Venus is believed to have suffered this fate.

The third ingredient needed is an inert gas, and its role is subtle. On Earth, the primary inert gas is molecular nitrogen. It does not contribute to greenhouse warming, because a nitrogen molecule has an even distribution of electric charge across it. Quantum physics tells us that such molecules are largely incapable of absorbing radiation. Counterintuitively, despite being the dominant gas by mass, molecular nitrogen is transparent to the radiation received and emitted by Earth. However, as the atmosphere warms and accumulates water vapor, water and nitrogen molecules collide. Absorption of units of light or radiation, known as photons, must match the discrete energy levels within a water molecule. When water and nitrogen molecules collide, deficits or surpluses of energy are exchanged. Known as pressure broadening, this effect increases the extent to which the water molecules may absorb radiation. Molecular nitrogen does not directly absorb light, but it influences how the greenhouse gases do so.

Inert gases also set a characteristic distance in the atmosphere known as the pressure scale height, which determines whether an atmosphere is puffy or compact. Hydrogen-dominated atmospheres tend to be puffier than their nitrogen-dominated counterparts. Furthermore, inert gases may participate in the chemistry involving greenhouse gases and can alter their abundances.

If we were to move Earth farther from the Sun, then at some point carbon dioxide would condense out of its atmosphere. As this greenhouse gas was removed, the atmosphere would cool and the overall temperature would drop. The outer boundary of the habitable zone is the distance from the Sun at which the atmosphere becomes too cool to support liquid water on the surface of the body. At high pressures, nitrogen molecules may form transient pairs, which have an uneven distribution of electric charge across them. These pairs produce a weak greenhouse effect known as collision-induced absorption. One imagines that the loss of gaseous carbon dioxide may be compensated for by packing more molecular nitrogen into the atmosphere, but there is a limit to the mileage gained, because the nitrogen also condenses out, at some point, when the temperature becomes too low.

Once we understand how greenhouse gases control the habitable-zone boundaries, we may imagine different flavors of habitable zones. Molecular nitrogen may be swapped out for molecular hydrogen, which has a considerably lower condensation temperature: tens of kelvin, rather than about a hundred. For planets with hydrogen-rich atmospheres, the outer boundary of the habitable zone may extend several times as far from the star, because molecular hydrogen compensates for the loss of the incondensible greenhouse gas through collision-induced absorption, thereby warding off its condensation. Water and carbon dioxide may be exchanged for other greenhouse gases, which could absorb and reradiate heat at other wavelengths or frequencies. Generally, a greenhouse gas is effective only if it is absorbent at wavelengths over which the planet is emitting radiation. A greenhouse gas that favors the absorption of blue light is useless if the planet emits only red light.

A fascinating example of a place with alternative atmospheric chemistry is found on Titan, a moon of Saturn that is about 40 percent of the size of Earth and has a fully functioning atmosphere. As in Earth’s atmosphere, the inert gas is molecular nitrogen, and methane is a greenhouse gas. But unlike on Earth, where methane exists only in gaseous form, it is a condensible greenhouse gas on Titan because of the considerably lower temperatures. Instead of carbon dioxide, the incondensible greenhouse gas is molecular hydrogen, which plays a negligible role on Earth. Molecular hydrogen warms the atmosphere of Titan via collision-induced absorption. Titan is hardly in the habitable zone for liquid water, but it would be in the habitable zone for liquid methane!

Without knowledge of the major molecules of an exoplanet’s atmosphere, we can only speculate whether it resides in the habitable zone for liquid water. It is akin to assuming that the exoplanet has an atmosphere exactly like Earth’s, consisting of nitrogen, water, and carbon dioxide—in precisely the same relative amounts, summing up to exactly the same total mass. Declaring a freshly detected exoplanet to be in the “habitable zone” amounts to little more than media spin if its atmospheric composition is unknown. Even professional astronomers sometimes forget this fact.

One of the most promising worlds in which to search for life in our Solar System illustrates why the habitable zone concept may be incomplete. Europa, one of Jupiter’s moons, sits outside of the traditional habitable zone. It has no atmosphere, and water is not liquid at its surface. However, a body of evidence suggests that a deep ocean exists beneath its icy surface, which may host life. Unfortunately, even if subsurface habitats for life are common on exoplanets, they are currently invisible to astronomers.

Thinking Outside the Zone

Current technology largely restricts us to characterizing the atmospheres of exoplanets that are Jupiter-like in size. As technology advances, astronomers expect to decipher the atmospheres of smaller, Earth-like exoplanets. Can we predict what the compositions of these atmospheres are in advance? Unfortunately, this task is daunting for smaller exoplanets. We expect the gas-giant exoplanets to have volatile elements (ones that vaporize at modest temperatures) in a mix that bears some semblance to those of their parent stars. For exoplanets dominated by a rocky core, we expect their refractory elements (ones that remain solid), but not the volatile ones, to mirror those of the star. In other words, we expect the rocks of the exoplanet, but not its gas, to mirror the metals in the star.

This expectation is certainly met by Earth, whose nitrogen-dominated atmosphere hardly resembles the hydrogen-dominated Sun. On Earth, the amount of carbon dioxide present in the atmosphere is regulated by the inorganic carbon cycle, which operates on geological time scales of hundreds of thousands of years. Through the process of weathering, gaseous carbon dioxide reacts with silicate rocks and water to form calcium carbonate, which is then subducted into Earth’s mantle. This part of the cycle acts as a carbon sink. Carbon dioxide is released back into the atmosphere via outgassing and volcanic activity. The inorganic carbon cycle acts like a geochemical thermostat: Weathering is more active when the conditions are wetter and warmer, which regulates the amount of carbon dioxide present (although not on short enough timescales to mitigate human-induced climate change). And when the Earth is in a frozen, ice-covered state, weathering is shut off. Outgassing continues to increase the amount of carbon dioxide in the atmosphere of this snowball Earth, until greenhouse warming suffices to melt the ice and snow.

The existence of the inorganic carbon cycle on Earth suggests that to understand the atmospheres of rocky exoplanets we need to understand the geochemistry of their surfaces. Is water always the solvent? Are the minerals and rocks the same as those on Earth? Is carbon dioxide the only greenhouse gas being geochemically regulated? Are these long-term geochemical cycles necessary for stable, habitable climates? Until we resolve these puzzles, our theories will have little predictive power.

That said, nature offers hints that life elsewhere in the Universe, if it exists, may not be that different from life on Earth. Science fiction has popularized the idea of silicon-based life, since silicon resides in the same group as carbon in the periodic table. This analogy breaks down when one examines the details, however. The chemical bond of silicon dioxide is too strong, while the silicon–silicon bond is too weak. Silicon dioxide (quartz) is an overly stable sink of silicon and is insoluble in water. These properties prevent silicon from forming a variety of complex molecules as carbon does. This expectation is consistent with what astronomers find when pointing their telescopes at seemingly uninteresting parts of space: an abundance of organic molecules, ranging from methanol and glycine (an amino acid) to fullerene (the “buckyball” with 60 carbon atoms). The building blocks of life, as we understand them on Earth, are commonly found elsewhere in the cosmos—preassembled. Darwin’s “warm little pond” idea of organic molecules forming in a primeval soup on Earth may need rethinking.

We are hunting for something we cannot yet sharply define.

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In the future, as astronomers detect molecules in the atmospheres of Earth-like exoplanets, the challenge will lie in interpretation of the data. What combinations of molecules need to be present for us to declare that extraterrestrial life has been detected? Classic ideas include the presence of oxygen and ozone. A potential false positive is the abiotic production of oxygen and ozone via the photolysis of water—the breakup of water molecules when they are exposed to ultraviolet radiation from the star. On Earth, water resides close to its surface, rendering such a process ineffective. The laws of physics, as we understand them, suggest that such a cold trap may not be operational on all exoplanets, implying that the detection of oxygen and ozone alone cannot be robust indicators of life. (We may potentially distinguish this scenario by measuring the escape of hydrogen from the exoplanet.) We need to understand what we should additionally look for.

Indicators of Life

Another consideration in the search for life on exoplanets is that our candidates for biosignature gases are based on the atmosphere of Earth and on metabolic cycles as we understand them. It is conceivable that some fraction of rocky exoplanets instead have thin, hydrogen-dominated (rather than nitrogen-dominated) atmospheres. In such atmospheres, atomic hydrogen acts as a radical (a reactive state in which some electrons are left unpaired) and destroys most of the molecules we regard as indicators of life. Ammonia and nitrous oxide are the most promising candidates for biosignature gases in such environments, because they are spared destruction by hydrogen. Methane and hydrogen sulfide, which are produced by life on Earth, become unreliable indicators, because they may be produced abiotically via geochemistry. Clearly, whether a specific molecule can be interpreted as a biosignature gas depends on the type of atmosphere the exoplanet has.

Generally, the difficulty with making the leap from the detection of molecules in an exoplanetary atmosphere to the identification of life is that many of the gases emitted by life are also manufactured by geology. The challenge may be framed as the identification of true biosignature gases in the face of geological false positives. Familiar gases such as ammonia, carbon dioxide, methane, oxygen, and water vapor are not uniquely associated with life. Exotic ones, such as dimethylsulfide, may potentially serve as biosignature gases, but they are difficult to detect in the spectrum of an exoplanetary atmosphere because their spectral signatures are subtle.

Ultimately, the search for life elsewhere in the Universe may require that we be able to define what life actually is. There is a lesson from earlier periods of human history—for instance, water was once described by its properties, rather than by its stoichiometry, due to our ignorance (then) of chemistry. We are facing the same struggle with the definition of life, because of our current inability to frame biology in sufficiently precise terms. Astronomers will continue to improve and sharpen their search for life elsewhere, while awaiting a general definition of life—a task best left to the biologists, but one with vast cosmic implications.